In a basic refrigeration system, a compressor is used to set a specific refrigerant pressure at the inlet side of the compressor. This is called the suction pressure and all associated connections are referred to as suction side connections. As a byproduct of setting the suction pressure, a refrigerant pressure is also established at the outlet side of the compressor. This is called the head pressure, with all associated connections referred to as head side. Since the head pressure is the direct result of refrigerant compression, the head side refrigerant is at a significantly higher pressure and temperature than the suction side.
In the basic system, the head side is connected to a condenser which reduces the temperature of the pressurized refrigerant in order to condense the refrigerant back into a liquid. This high pressure liquid is then supplied to an expansion valve which meters this refrigerant into an evaporator as a mixture of vapor and liquid. The evaporator typically is a finned tubular assembly which is built directly into a refrigerated fixture or case. In the evaporator the liquid refrigerant evaporates while absorbing heat from the evaporator surroundings. Within the fixture, fans create a circular flow of air past the products to be refrigerated and through the evaporator. In this manner, air warmed by the products within the fixture is subsequently cooled as it passes through the evaporator. To physically complete the system, the outlet of the evaporator is connected directly to the suction side of the compressor. The evaporator could also cool water which is then used for cooling purposes.
One characteristic of the metering process, where refrigerant is supplied into the evaporator via an expansion valve, is that the refrigerant experiences a significant pressure drop since the orifice of the expansion valve is significantly smaller than the cross section of the evaporator. Thus, the refrigerant pressure within the evaporator is effectively set by the suction side and not the head side of the system. Since a refrigerant's evaporation pressure and temperature are directly linked in a one-to-one manner, it is apparent that any device which controls the suction side pressure at the evaporator will directly control the evaporation temperature of the refrigerant within that evaporator and thus the minimum temperature which can be obtained within a circuit defined as a collection of refrigerated fixtures or cases having evaporators being interconnected and sharing a common evaporation pressure. One or more refrigerant compressors sharing a common suction side and a common head side is referred to by those skilled in the pertinent art as a rack. The head sides of compressors forming a rack are also associated with a refrigerant condenser.
In a typical supermarket, it's necessary to maintain different product groups at different temperatures (examples would include ice cream, frozen food, fresh meat, dairy, and produce). Since the quantity of each product type typically requires multiple refrigerated fixtures hereinafter referred to as cases, these cases are connected together to create circuits. In a circuit, a collection of refrigerated cases share a common supply and return piping for the required refrigerant. However, since rack systems which supply the refrigerant are costly, it is common to implement a reduced number of racks (e.g., a minimal configuration would consist of one rack, but typically includes one low temperature rack and one medium temperature rack). Consequently, head side connections from a single rack are via a condensor associated with the rack connected to various circuits which operate at different temperatures. One common solution to achieving unique circuit temperatures is to install mechanical Evaporator Pressure Regulator (EPR) valves on the return line from each circuit prior to connection to the suction side of the rack. Each evaporator pressure regulator valve, usually set by a refrigeration mechanic when the system is initially commissioned, works to establish a specific evaporation pressure, and thus a specific temperature, within the evaporators of the associated circuit. However, since the initial setting does not typically change, this approach prevents any type of dynamic system response and thereby dynamic regulation of circuit temperature necessitated by seasonal changes, improperly or overloaded cases and the like. In lieu of mechanical valves, electronically controlled EPRs, or EEPRs, are the preferred solution since an associated control algorithm may be implemented to achieve the desired circuit temperature regardless of fluctuations in either the store environment or the performance of the racks themselves.
One known method of controlling a multi-circuit refrigeration system is to select a lead circuit from a plurality of circuits, each including at least one refrigeration case. The lead circuit being defined as the circuit having the lowest temperature set point. A suction pressure set point for a compressor rack is initialized based upon the identified lead circuit. Changes in suction pressure set point are determined based on measured parameters from the lead circuit. The suction pressure set point is updated until the EEPR for the lead circuit is approximately 100% open. A problem associated with this type of refrigeration system is that the lead circuit will not necessarily be the circuit requiring the lowest suction pressure. For example, in a situation where a popular item is located in the refrigeration cases of a particular circuit, because of high turnover, that circuit may require more refrigeration, i.e. be more loaded, than other circuits in the system having lower temperature set points. In this case a lower suction pressure will provide the necessary additional refrigeration. In addition, poor case design in a circuit can also lead to heavy loading. If such a poorly designed circuit should be able to maintain the intended temperature, the circuit would require a low suction pressure to deliver the required refrigerating capacity. Another example is that different types of cases can have different energy efficiencies. Moreover, goods might accidentally be stacked wrongly in a case so that the refrigerated airflow from the evaporator does not flow correctly, whereby the refrigerating capacity is lowered. This results in a more loaded case. Accordingly, if the circuit having the lowest temperature set point does not correspond to the circuit under greatest load, a system configured in the above-described manner will not operate properly.
Based on the foregoing, it is the general object of the present invention to provide a method for controlling a refrigeration system that overcomes or improves upon the problems and drawbacks of the prior art.